Merge branch 'develop' into feature/test-nan-op-lt
added NaN test for logical_lt
add NaN test for operator_lt
added NaN test for op_lt
added doc for operator_lt
Merge branch 'develop' into feature/test-nan-op-lt
Merge pull request #1010 from stan-dev/feature/test-nan-op-lt

Feature/test nan op lt